When a digital signal is transmitted from one system to another, such as over a cable or wirelessly, the digital signal becomes distorted due to noise and attenuation. To recover the digital signal for processing by the receiver, a digital signal detector is used. In a typical digital signal detector, the incoming “dirty” signal is compared to a threshold level, where the threshold level may be an average level detected over a relatively long period of time, and a logical one or zero is determined by whether the data signal is sufficiently higher than or lower than the threshold level. In other systems, the threshold is fixed.
In another application, a D-flip flop is used to create a clean digital signal, where the D-flip flop receives a clock signal and a data signal and latches in the data signal (above or below a fixed threshold) when the clock transitions high or low.
In many situations, it is desirable to provide DC isolation between two circuits, where the two circuits are from different systems. This is also referred to as AC coupling. In one example, a digital receiver head end, which receives a raw digital signal from a cable or wirelessly, is DC isolated from a detector circuit that determines whether the signal is a logical one or a logical zero. Since the DC offset of a digital signal is not relevant to the information being transmitted, such a DC offset can be filtered out. One type of DC filtering is a high pass filter formed of a capacitor in series with the signal path and a resistor connected between the downstream terminal of the capacitor and ground. The RC time constant of the filter determines the attenuation of the signal at a certain frequency. The DC component is totally blocked by the filter.
One problem with such DC isolation of the detector circuit is that some applications entail very low frequency pulses which must not be filtered. For example, a GPS system generates a 1 pulse per second (1 pps) signal that needs to be accurately received. If the 1 pps signal were needed to pass through the high pass filter while the filter blocked a DC level, the filter would require a very large filtering capacitor and/or a high value resistor to create a sufficiently high RC time constant (>>1 second). Such a large capacitor is impractical, and using such a high value resistor would not maintain signal integrity.
In some applications, a digital signal is transmitted over a low cost cable whose attenuation is highly dependent on frequency. A digital signal is composed of a wide range of frequencies having certain relative amplitudes, and recovering the digital signal may require an equalizer in the receiver that compensates for the frequency-dependent attenuation by the cable. Such an equalizer requires customized adjustment for each type of cable used.
What is needed is a digital signal detector that uses AC coupling to block DC, yet is practical for detecting a low frequency digital signal and is relatively insensitive to frequency-dependent attenuation by a cable.